With the advent of a deep reactive ion etching (DRIE) process for forming slots and trenches in a semiconductor substrate, greater precision and control over the etching of silicon substrates in higher speed processes has been obtained. DRIE is a dry etching process carried out under high vacuum by means of a chemically reactive plasma, wherein the constituents of the plasma are selected in congruence with the substrate to be acted upon. Before the adoption of DRIE techniques to form trenches or slots in semiconductor substrates, most trenches or slots in substrates greater than about 200 microns thick were formed by mechanical blasting techniques or chemical wet etching techniques. However, such mechanical techniques or chemical wet etching techniques are not suitable for newer products which demand higher tolerances and smaller trenches and/or slots. DRIE enables deep anisotropic etching of trenches and slots with greater tolerances and without regard to crystal orientation.
DRIE techniques have progressed incrementally towards a goal of etching high aspect ratio features in semiconductor substrates wherein the aspect ratio is on the order of 1:100 width to depth. Hence, much progress has been made in forming vertical conduits or trenches with substantially perpendicular walls. The process scheme for achieving high aspect ratio slots or trenches in semiconductor substrates includes a series of sequential steps of alternating etching and passivation. Such aniosotropic etching techniques are described in U.S. Pat. Nos. 5,611,888 and 5,626,716 to Bosch et al. the disclosures of which are incorporated herein by reference.
A schematic diagram of a DRIE system 10 is illustrated in FIG. 1. The system 10 includes a ceramic reaction chamber 12 and a radio frequency (rf) unit 14 for providing source power to a coil 16 to generate a plasma in the reaction chamber 12. A wafer 18 containing a plurality of semiconductor substrates is disposed in the chamber 12 on a cooled chuck which is part of platen 20. The temperature of the platen/chuck 20, and thus the wafer 18, is selected on a chiller unit 22 providing helium gas to the platen/chuck 20. A platen power unit 24 provides rf biasing power to the platen 20 during the etching process. The chamber 12 is maintained at a low pressure during etching by a vacuum pumping unit coupled to a vacuum port 26. A reactive gas is introduced into the chamber through a gas inlet port 28. A bellows system 30 may be provided to adjust a height of the platen 20 before the etching process.
Accordingly, most dry etching systems 10 are designed to etch substantially vertical wall slots and trenches in the substrate 18, i.e., walls that are substantially perpendicular to a surface of the substrate 18. However, for micro-fluid ejection heads, it has been found that substantially vertical walls may entrap more air in fluids passing through relatively narrow slots. Such air entrapment can lead to fluid starvation for ejection devices on a device surface of the substrate. Accordingly, there is a need for improved DRIE techniques to form fluid feed slots having reentrant walls in micro-fluid ejection head substrates.
With regard to the foregoing, there is provided a method of micro-machining a semiconductor substrate to form through fluid feed slots therein. The method includes the steps of providing a semiconductor substrate wafer having a thickness greater than about 500 microns and having a device side and a back side opposite the device side. The back side of the wafer is mechanically ground to provide a wafer having a thickness ranging from about 100 up to about 500 microns. Dry etching is conducted on the wafer from a device side thereof to form a plurality of reentrant fluid feed slots in the wafer from the device side to the back side of the wafer.
In another embodiment there is provided a structure for a micro-fluid ejection head. The structure includes a semiconductor substrate having a device side and a back side and containing one or more fluid feed slots dry etched therein from the device side to the back side. The fluid feed slots have a substantially reentrant wall profile and the slots are devoid of substantially vertical wall portions adjacent the back side of the substrate.
An advantage of the exemplary process disclosed herein can be that the process is capable of providing precisely formed slots having a reentrant profile in semiconductor substrates. Such slots can have reentrant profiles that are substantially devoid of vertical wall sections and that have lower incidence of top side damage. Accordingly, superior reentrant slot profiles can be formed in semiconductor substrates that provide improved fluid flow properties for fluids flowing through the slots.